Pituitary glands obtained at autopsy of 125 women with disseminated breast carcinoma were studied to determine whether pituitary prolactin cell abnormalities (hyperplasia or adenoma) might be involved in the pathogenesis of breast carcinoma. In addition, we studied 85 pituitary glands obtained from unselected, consecutive autopsies in women without breast carcinoma but who died of other diseases (control group). The frequency of lactotroph hyperplasia was slightly higher in patients with breast carcinoma than in the control group, but the difference was not statistically significant, nor were differences in the frequency and size of pituitary adenomas, prolactin-producing or otherwise. No correlation was found between the presence of lactotroph hyperplasia or prolactin-producing adenomas (or both) and such factors as the patient's age, bilaterality of the carcinoma, previous treatment with tamoxifen citrate or oophorectomy, stage of disease, or survival. The frequency of breast carcinoma metastatic to the pituitary gland was higher in the study group than in the control group; however, the difference was not statistically significant. No preferential site of metastatic involvement in the pituitary gland was noted. Relative proportions of other lesions such as infarcts, cysts, lymphocytic infiltrates, and basophilic invasion were similar in the study and control groups. This study indicates that accumulation of prolactin cells, whether hyperplastic or adenomatous, cannot be considered a major risk factor for the genesis or progression of breast carcinoma.